The present invention relates to an objective lens to be fitted at the distal end of an endoscope, more particularly, to a wide-angle endoscopic objective lens that can provide a view angle (a range in which an image can be focussed by the lens) greater than 50.degree..
Endoscopes are so constructed that a real image created with an objective lens fitted at the distal end is transmitted through image transmission optics to a position convenient for observation. A suitable image transmission optics is selected depending on use from among various types of fiber bundle and relay optics consisting of an ordinary lens and a gradient index lens.
From the viewpoint of use in connection with the endoscope, the objective lens at the distal end of the endoscope is required to be small in diameter and yet have an ability of forming an in-focused image over a wide view angle. An optics comprising a plurality of spherical lenses is capable of providing a wide visual field while effectively correcting various aberrations. However, the use of a plurality of spherical lenses having an outer diameter smaller than 1 mm results in extremely increased cost associated with difficulty in lens polishing, assembling and adjusting operations.
To overcome this problem, a rod lens having a gradient refractive index in a radial direction is proposed as a low-cost, small-diameter objective lens. The rod lens can be manufactured easily and at low cost by ion-exchange and other techniques to have a diameter not larger than 1 mm. Further, since the rod lens is planar on both sides, the rod lens offers the advantage of great simplicity in polishing both end faces, assembling into an endoscope and achieving alignment of optical axes.
The refractive index of a rod-shaped, gradient index lens is expressed as EQU n(r).sup.2 =n.sub.o.sup.2.multidot.{1-(g.multidot.r).sup.2 +h.sub.4 (g.multidot.r).sup.4 +h.sub.6 (g.multidot.r).sup.6 +h.sub.8 (g.multidot.r).sup.8 + . . . }
where
r: the distance from the optical axis PA0 n(r): the refractive index at distance r from the optical axis PA0 n.sub.o : the refractive index on the optical axis PA0 r.sub.0 : the effective radius of the gradient index lens PA0 g: the gradient index coefficient (2nd order) PA0 h.sub.4, h.sub.6, h.sub.8, . . . : the gradient index coefficient (higher order). PA0 r: a distance from the optical axis PA0 n(r) a refractive index at distance r from the optical axis PA0 n.sub.o : a refractive index on the optical axis PA0 r.sub.o : an effective radius of the gradient index lens PA0 g: a gradient index coefficient (2nd order) PA0 h.sub.4, h.sub.6, h.sub.8, . . . : a gradient index coefficient (higher order) PA0 r: a distance from the optical axis PA0 n(r) a refractive index at distance r from the optical axis PA0 n.sub.o : a refractive index on the optical axis PA0 r.sub.o : an effective radius of the gradient index lens PA0 g: a 2nd order gradient index coefficient PA0 h.sub.4, h.sub.6, h.sub.8, . . . : a higher order gradient index coefficient. PA0 n: the homogeneous refractive index of the planoconvex lens PA0 R: a radius of curvature R of the convex surface of the planoconvex lens.
The peripheral portion of the gradient index lens usually has a great departure from the design value of gradient refractive index, and thus cannot be substantively used as a lens. In some cases, the peripheral portion of the lens is made opaque in order to prevent the stray light due to the reflection from the lateral surfaces of the lens. Hence, the range of the gradient index lens that has a sufficiently good gradient reflective index to contribute to the formation of an in-focused image is defined as the effective radius r.sub.o. The effective radius r.sub.o of a rod lens is not necessarily the same as its apparent radius.
If the view angle of a rod-shaped, gradient index lens used as an objective lens is written as .theta., the following relation holds EQU .theta.=n.sub.o.multidot.g.multidot.r.sub.o (rad)
Note that the objective lens designated later as Comparative Example (see FIG. 17) has an view angle .theta. of 38.7.degree..
If an objective lens having a wide visual field is used with an endoscope, the subject can be seen through a correspondingly wide range, facilitating various operations with the endoscope. In order to increase the view angle .theta. of the rod-shaped, gradient index lens, the value of n.sub.o.multidot.g.multidot.r.sub.o, hence, the difference in refractive index between the center and the periphery of the lens, must be increased. However, it is generally held that the maximum value of n.sub.o g.multidot.r.sub.o for the rod-shaped, gradient index lens that can be easily attained by the ordinary ion-exchange technology in current use is no more than about 0.70. In other words, a wide-angle rod-shaped, gradient index lens having a view angle .theta. in excess of 0.7 rad (i.e. about 40.degree.) is considerably difficult or troublesome to manufacture.